In conventional digital-to-analog converters, a first conversion stage is followed by multiple stages of low-pass filtering to filter out unwanted noise. In one type of digital-to-analog converter, a digital delta-sigma modulator is utilized. The delta-sigma modulator receives a digital input and converts it into a one-bit digital output. This output is typically passed through a one-bit DAC and then into an active RC low pass filter. The active RC low pass filter utilizes a series of resistors and various active components to realize the desired filter function. The disadvantage to this type of filter is the sensitivity of the filter to variations in the components. A significant amount of trimming is often required.
A problem exists with conventional digital-to-analog converters utilizing any type of switched capacitor filter is the requirement for conversion from the sampled data domain on the output of the switched capacitor filter to a continuous time domain. In the past, an active filter has been utilized to directly convert between the sampled data output by the switched capacitor and a continuous time output. However, this conversion typically results in a high degree of distortion that is added to the signal. Any distortion is detrimental to the overall performance.
One method proposed to reduce this distortion can be found in related U.S. Pat. No. 5,198,782, entitled, "Switched-Capacitor Filter with DAC Input," U.S. Pat. No. 5,239,210, entitled "Low Distortion Unity Gain Amplifier for DAC" and U.S. Pat. No. 5,198,782, entitled "Low Distortion Amplifier Output Stage for DAC," all of which are incorporated by reference. This method is operable to convert the digital input signal to a sampled analog data signal and then processed it with a sampled data/continuous time buffer of the type illustrated in U.S. Pat. No. 4,698,596, FIG. 8. However, it is not easy to incorporate the D/A architecture into this buffer.